This invention generally relates to remote control receivers and more specifically is directed to an infrared (IR) remote control detector/decoder providing improved noise immunity particularly adapted for use with a television receiver.
Television receiver remote control systems generally employ either an ultrasonic or optical link. Spurious signal problems associated with ultrasonic remote control systems, such as multi-path reception, telephone ringing, key jangling, etc., limit the practicality of these systems for home use and have led to wider use of IR remote control systems. However, even systems utilizing signals propagated in the IR spectrum suffer from spurious inputs resulting in control signal input errors.
One approach to reducing erroneous inputs in a remote control system in which light signals are used as the remote control signal is disclosed in U.S. Pat. No. 3,928,760 to Isoda. This approach utilizes a light signal emitted from an electroluminescent diode which is amplitude-modulated by a modulated frequency in the ultrasonic frequency range. By thus amplitude modulating a light signal at an ultrasonic frequency, ambient noise in the ultrasonic frequency spectrum, such as that previously described, is allegedly avoided. In addition, it is asserted that by modulating the light signal at a relatively high frequency, amplitude-modulated light noise generated by various sources, such as fluorescent lamps, is avoided.
An extension of the approach employed in the above referenced patent is disclosed in U.S. Pat. No. 3,866,177 to Kawamata et al wherein is disclosed a remote control system using light as a controlling signal which is first modulated by a high frequency signal and is then further modulated with a low frequency signal. The received light signal is demodulated to pick up the low frequency component which serves as the control signal. The purpose of this high and low frequency modulation of the remote control light signal is to avoid spurious inputs generated by DC energized fluorescent lamps in which the DC current is transformed into a high frequency current of 15 kHz to several tens kHz by a high frequency inverter and energizes the lamp thereby. The high frequency signal thus generated is frequently not stable further reducing the desirability of a light signal remote control system modulated at a high frequency.
In an attempt to make the remote control system more noise immune, the control signal is propagated in the form of various combinations of pulses of a single frequency wave. With the number of pulses being preset in the transmitter and with the receiver responsive to only a predetermined number of pulse arrangements, such remote control systems proved to be not only more noise immune, but also capable of transmitting a large number of commands for controlling various functions of a television receiver. However, even with the use of complex pulse code schemes, remote control systems utilizing this approach offer only limited noise immunity.
U.S. Pat. No. 4,121,198 to Tsuboi et al represents an attempt to further improve the noise immunity of a pulse code modulation remote control system. In this approach a coded command signal transmitted as a plurality of pulses of a first frequency F.sub.1 also includes additional pulses of a second frequency F.sub.2 inserted in spaces between the pulses of the first frequency. System noise immunity is allegedly improved by making the receiver insensitive to noise received during the inter-pulse periods. However, this approach requires the use of sophisticated pulse processing circuitry because two pulse frequencies are utilized and is limited in the number of pulse codes available since one half of the available pulse codes must be devoted to filling in the spaces between the control signal pulses operating at a frequency of F.sub.1.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a remote control system in which a pulse code modulated control signal is used to modulate a second signal so as to transmit the control signal in a more noise free portion of the IR spectrum. The pulse code modulation (PCM) approach provides for an increased number of available codes and associated television receiver control functions.